Yesterday, we chatted with dietician Kara Mitchell about the importance of knowing your Resting Metabolic Rate, the number of calories your body needs to carry on such basic functions as breathing, pumping blood, growing new cells. Go below that number and your body will think it’s in trouble, switch into survival mode and burn fewer calories to perform such tasks.

That physiological fact is important to people trying to lose weight because going below your RMR won’t help you shed pounds. Your body will go into primal panic mode and simply perform the same functions, perhaps not as efficiently, on fewer calories. And that will make you cranky. Stick to your RMR and any additional activity your body performs — taking the stairs rather than the elevator, parking in the spot farthest from the front door, walking for half an hour at lunch — will go toward burning fat and building calorie-consuming muscle.

Alas, your RMR is a pretty Spartan number. Say you’re a 40-year-old 5’ 4” woman who weighs 135. According to your Resting Metabolic Rate, you need 1,263 calories a day to  perform your basic body functions. Breakfast and lunch from a fast food joint would pretty much tap out your RMR by noon. That convenience factor coupled with a lack of time is the biggest challenge to good eating that Mitchell sees with her clients.

“It often boils down to time, which leads to a broader discussion about priorities,” says Mitchell. “We try to break through some of the myths about finding enough time to eat right.”

With that in mind, she offers these tips:

• Do what you can to avoid those fast food meals. That Egg McMuffin combo (with hash browns, ketchup and a small Coke) you grabbed so you could sleep in another 15 minutes? 575 calories. Wake up 15 minutes earlier and have some oatmeal (307 calories), which will stick with you at least until lunch and offers a variety of other nutritional benefits (low in saturated fat, very low in Cholesterol and Sodium, good source of dietary fiber, phosphorus, selenium, manganese).
• Embrace beans, rice, pasta. All can be prepared quickly, all are good sources of nutrition.
• Embrace frozen vegetables. They can be plucked from the freezer and prepared quickly. And, says Mitchell, contrary to what you may have heard, frozen vegetables usually have better nutritional value than fresh. “Vegetables begin losing nutrients the moment they’re picked,” says Mitchell. “Since most frozen vegetables are processed near where they’re picked, they generally retain more of their nutritional value.” (Canned vegetables, on the other hand, are often packed in sodium-rich brine, only about 60 percent of which is removed by rinsing, she says.)
• Embrace leftovers. Make more for dinner than you plan to eat and take the leftovers for lunch.
• Eschew processed foods. They aren’t as “calorie dense,” says Mitchell, meaning you won’t get as much bang for your RMR-restricted calories. And they frequently come packed with chemicals your body doesn’t need.
• Make your own fast food. We resort to fast food because it’s … well, fast, and we’re running late and if we’re not at the desk by 8 a.m. sharp Mr. Dithers will have a conniption fit. So, says Mitchell, make your own, healthy fast food. “Go shopping on the weekend and make your lunches Sunday night,” she advises. “Some foods are more sensitive to time,” she acknowledges. For instance, you wouldn’t want to make a PBJ Sunday night for lunch Thursday. A pasta dish, on the other hand, should keep well.
• Embrace precut veggies & fruit. You learn this back in preschool, that precut veggies and fruit make great snacks. Raw baby carrots, cherry tomatoes, sliced peppers, celery, cucumbers, broccoli, cauliflower all do well on their own, and do even better when jazzed up with a dip using fat-free sour cream as a base.

Maybe the most important thing to keep in mind in your efforts to eat more sensibly, says Mitchell, is not to take an all-or-nothing approach. You’re changing your lifestyle — not joining a cult. An early morning Egg McMuffin now and then won’t kill you.

“It’s OK to have a moderate approach,” says Mitchell. “What’s important is to make this a lifestyle change so you aren’t doing the same thing again next year.”

• Mitchell’s resource tip: Looking for online help to help get your diet in line? Mitchell says the USDA’s MyPyramid.gov provides nutritional information, recipes, weight loss advice and a slew of other helpful information.

Photo: Beans, rice, green, leafy vegetable — this plate’s got it all, baby!